1. Field of the Invention
The present invention relates to a sheet feeding apparatus for separating and feeding stacked sheets one by one and to an image forming apparatus such as copying apparatus or printer using the sheet feeding apparatus.
2. Description of the Related Art
An image forming apparatus such as copying apparatus or printer separates and feeds stacked sheets one by one and forms an image. However, in such an image forming apparatus, the operation is limited depending on a type of sheet which is fed. For example, in the case of a transparent sheet such as an OHP sheet, duplex printing cannot be performed. If the user erroneously selected the duplex printing, generally, the image forming apparatus stops the operation or controls so as to automatically perform simplex printing.
For this purpose, it is necessary to detect the sheet type. As a method of detecting the sheet type, for example, there is a method whereby a reflecting type sensor is used, the sheet type is detected by reflectance of the sheet, and the image forming apparatus is controlled based on a detection result (refer to Japanese Patent Application Laid-Open No. 2000-098813).
However, in the case of a construction using the reflecting type sensor as disclosed in Japanese Patent Application Laid-Open No. 2000-098813, there is a case where the reflectance changes due to a fluttering of the sheet at a position near the detecting position and the sheet cannot be accurately identified. There are many types of sheets such as, particularly, sheet whose surface has a glossiness like an OHP sheet, sheet whose surface has been coated with a film because of a reason for improvement of print quality or the like, and the like. There is, consequently, a case where even the OHP sheet is detected as a normal sheet.
Further, there occurs a case where a foreign substance or the like is deposited onto the sensor surface due to the passage of the sheet and an accurate output is derived.
A construction in which a reflecting type sensor is attached to a side regulating plate and a type of stacked sheet bundle is detected by the sensor has been disclosed in Japanese Patent Application Laid-Open No. 2002-139963. The sensor disclosed in Japanese Patent Application Laid-Open No. 2002-139963 is a reflecting type optical sensor having a light emitting element (not shown) for emitting light and a photosensing device (not shown) for receiving reflection light of the light emitted from the light emitting element. The reflection light from a top one of the papers stacked on a feeding tray is received by the photosensing device. The type of the paper is discriminated based on an output of the received light.
In the construction disclosed in Japanese Patent Application Laid-Open No. 2002-139963, when the top one of the papers stacked on the feeding tray that is the detection object is transparence, the light is irradiated onto a paper other than the top one of the papers stacked on the feeding tray. The photosensing device detects the reflected light from the paper other than the top one of the papers. Therefore a wrong detection might be done. In addition if a sensor discriminates the type of the paper after the paper is fed from the feeding tray, a certain discrimination might be not made because the position of the paper is not suitable for the sensor.